And as obesity figures continue to rise, that number is likely to grow even larger.

How did we get here?

Last week, at the 3rd International Congress on Physical Activity and Public Health, many researchers presented data tracking recent trends in the amount of leisure-time physical activity we’re getting, nowadays. But one of the keynote speakers, Dr. William Leonard of Northwestern University, presented an intriguing talk that took the long view.

The very long view.

Like, hundreds of thousands of years long.

Dr. Leonard is an anthropologist, you see. He talked about how humans evolved, specifically how how changes in our diet and activity level changed our body type.

Basically, said Dr. Leonard, as our brains got bigger, they placed larger demands on our metabolism, and our diets became more nutrient- and calorie-dense to support them.

Leonard proposed that the recent and much-talked about uptick in the calorie-density of our foods over the past few decades (higher fat content, larger portion sizes) is simply an extension of what’s been happening to us, on an evolutionary scale, for millions of years. But the difference is key: dietary changes that used to to take thousands and thousands of years to occur have happened within a single lifetime.

Even so, he suggested that we might be missing the real story by focusing so much on the increase in calories in our diets. In fact, he notes, while calorie content of the diet in the developed world has increased since the fifties, that increase leveled off in the 80s. Yet obesity rates continued, and continue, to rise.

To explain this, he suggests that it’s decreased calorie expenditure that plays a larger role in obesity than caloric intake.

Throughout our evolution, our caloric intake increased to match greater and greater needs we placed upon our bodies – hunting calorie-dense animals is more demanding than gathering low-calorie-dense crops. But this eon-old trend toward increased calorie-burning is now experiencing a dramatic reversal: Our jobs have become much more sedentary since the 80s and the advent of the computer.

Our bodies have evolved over millions of years to actively and effectively meet our caloric needs from the environment. But now we’re suddenly accelerating the trend toward more calories – and by changing the physical environment to make things easier, we’re reversing the evolutionary trend toward burning more calories.

The net effect: we’re upsetting an equilibrium our species has managed to maintain for thousands and thousands of years.